Integrated circuitry meant for interface with other types of circuitry, such as power transistors, etc., which may require various load requirements, requires output circuitry capable of interfacing with the various loads. An example situation occurs where the output current available from high side driver outputs is limited by the gain of the high side pass transistor(s). In order to guarantee a minimum specified output current, the circuit must be designed to operate under conditions of minimum gain. However, when a high gain transistor is encountered, output current can become excessive, resulting in high levels of power dissipation, and possible IC damage. To prevent IC damage, a means of limiting the maximum amount of output current is necessary.
Traditional methods of limiting output current employ a current sensing element placed in series with the output current path. This element is typically a resistor across which a sense voltage is developed, or a diode that is part of a current mirror configuration. The current through the current sensing element creates a voltage drop that subtracts from the available output voltage of the circuit. In circuits with low supply voltages, unnecessary voltage drops are not desired. Additionally, each output current path requires a current sensing element and the associated circuitry, requiring additional silicon area and additional cost for circuits with more than one output current path.
It has also been found that circuitry for implementation of the current sensing elements tends to have substantial temperature coefficients, causing changes in the limited output current with changes in temperature. Temperature compensation circuitry to eliminate this trait requires even more silicon area and more cost. Additionally, if the current sensing element is an integrated resistor, silicon processing variations result in variations in the value of the resistor, leading to variations in values of the output current limit level.